Traveling wave tube



United States, Patent O,

2,895,071 TRAVELING wAvn TUBE Rudolf Kompfner, Far Hills, NJ., assignorto Bell Telephone Laboratories, Incorporated, New York, N.Y., acorporation of New York Application December 23, 1952, Serial No.327,566

Claims. (Cl. S15-3.6)

This invention relates to electron discharge devices and moreparticularly to such devices of the traveling wave tube type whichemploy interdigital wave guiding structures as the wave interactioncircuits. Y

In traveling wave tubes, an electron stream is pro jected past a waveinteraction circuit along which is propagating an electromagnetic waveatA a velocity such that cumulative interaction results between theelectron stream and the electromagnetic wave whereby amplification isprovided to the electromagnetic wave. There have been devised hithertotraveling wave tubes suitable for opera tion in the millimeterwavelength region, but such tubes generally either have had lessfrequency range or lower efficiencies or have been more diiicult toconstruct than is desirable.

l Accordingly, objects of the present invention are to increase thefrequency range of operation, improve the eflil ciency, and facilitatethe construction of traveling wave tubes suitable for operation atextremely short wavelengths, such as in the millimeter wavelengthregion.

Another object is to extend the frequency range of traveling wave tubesinto still shorter wavelength regions.

To this end, a principal feature of the invention is an interdigitalwave guiding structure which includes a hol-` low wave guiding memberhaving as linger elements a longitudinal linear array of spaced wireloops extending into the hollow of the member, alternate loops beingconnected to the same surface of the member and adjacent loops beingconnected to opposite surfaces of the memJ ber in an interdigitalpattern. In a preferred embodiment of the invention, each of the wireloops is,hairpin or U-shaped having both its ends connected tothe hollowwave yguiding member, andan electron, stream is` pro-l jectedlongitudinally through a `region of overlap of adjacent loops. .Y

The use of interdigital wave guiding structures as wave transmissioncircuits in traveling wave tubes has been'V known hitherto but suchpreviouslyV known structures have not been especially suitable foroperation at extremely short wavelengths, such as inthe millimeterwavelength region. Such prior art structures have generally included, asfinger elements, arrays of stubs or vanes extending into the hollow of awave guiding member and usually formed as integral portions thereof. ASince the dimensions, of the finger elements are advantageously .of theorder of a quarter of the operating wavelength or ture of wire loopelements suitable for tubes adapted'to,

operate at a center band wavelength of 6 millimeters.

Moreover, these same techniques Fcan similarly be applied" to theconstructon of wave guiding structures for -tubes' Y for operation atstill shorter wavelengths. Since it is gen` I erally desirable ininterdigital structures of this kind to limit the length of the fingerelements to less than a quarter of the operating wavelength in the wave'guiding structure, it can be appreciated that the construction ofparable wavelengths would pose an almost insurmountable problem,particularly if it Were sought to providel tubes of reproduciblecharacteristics. The term length of the finger elements as used hereinrefers to the height of the U-shaped Enger elements rather than thedistance around said elements. Additionally, an interdigital circuit inaccordance with the invention can be operated at considerably higherefficiencies than comparable prior art structures since the loopconstruction facilitates access of the electron stream for travel pastthe regions of high electric ield. This factor is of particularimportance at the extremely short wavelengths where the relatively smalldimensions of the tube construction make high gain diiiicult ofattainment.

Moreover, although the invention will be described with specificreference to embodiments in which finger elements are U-shaped wireloops for use at millimeter wavelengths, for longer wavelengths thefinger elements may be U-sha'ped tubing through which can be circulateda cooling fluid for operation at high power levels.

The use of interdigital-type wave circuits is of special importance intubes to be used in spatial harmonic type operation, where the electronbeam is in synchronism with a spatial harmonic component of thetraveling wave which is propagating with a much slower phase velocitythan the fundamental component of the traveling wave. In a preferredembodiment of the invention to be described below, an interdigital wavecircuit utilizing hairpin or U- shaped wire fingers is incorporated in abackward wave oscillator in which an electron stream is in synchronismwith a forward-traveling spatial harmonic of a wave whose fundamentalcomponent is propagating in a back- Ward-traveling direction (ie. adirection opposite to that` of the electron stream).

The invention will be better understood from the following more detaileddescription taken in conjunction with the accompanying drawings inwhich: Y

.Fig. lshows schematically a longitudinal cross section of a backwardwave oscillator asv an illustrative embodiment of the invention;

Fig. 2 shows a transverse cross sectionttaken along the Fig. 3 showsschematically in longitudinal cross section an amplifier suitable foroperation either in a forward-or backward wave mode as an alternativeembodiment of "the invention.

show a backward wave oscillator which is enclosedby an* evacuated glassenvelope 10. At opposite ends of -the envelope are positioned anelectron gun 11 and a collector electrode 12 in target relationship. Theelectron gun 11 v is of standard design and comprises essentially acathode 13, a beam focussing and intensity control electrode 14, and anaccelerating anode 15. u prises a cup-shaped element with its open endyfacing the electron gun. Interposed between the electron gun andthecollector is the structure 16, of a non-magnetic metal such as copper,and comprising an output section 17 and an elongated interaction section18. The interaction section 18 has a hollow interior, preferably ofrectangularl cross section, which serves as a wave guiding passage 19therethrough. The cross-sectional dimensions of wthe pas- -sage 19decrease graduallymfrom maxima at the electron Y. vPatented July14,719.59,

The collector 12 corn' source end to smaller Values which are uniformalong the major portion of its length. This hollow interior is alignedwith the path of electron ow between the electron gunand `the collectorelectrode and the structure 16 is provided with suitable apertures 2()and 21 at opposite ends for traversal of the electron tlow axiallytherethrough. For backward wave oscillator applications, the collectoror downstream end of the passage is .closed off except for the aperture21. Throughout the specification andthe claims, the term downstream isused to 'denote a point or location closer to the collector than thepoint or location with which it is being compared. Conversely, the termupstream denotes a point or location closer to thesgun than .the pointwith which it is being compared. In order to make this end substantiallyretlectionless over a broad band of wavelengths, in the passage there isinserted a tapered block or wedge 22 of dielectric with its tapered face23 coated with lossy material `such as powdered graphite. This wedgesubstantially ills the wave guiding passage and is apertured for passageof theelectron ilow. At the electron source or upstream end, the waveguiding passage 19 merges into a wave guiding passage 24 formed as theinterior of output section 17 of the structure 16. This wave-guidingpassage 24 similarly is of rectangular cross section for matching thecross section of the wave guiding passagef19 at the point of merger. Forconvenience in leading off the abstracted enero a conventional rightangle bend Z is provided between the wave guiding passages 19 and 24.Additonally a hollow Wave guide 26 of rectangular cross section andsuitably tted to the output section 17 forms a continuation of the waveguiding passage 24 to the wall of the `glass envelope where it iscapacitively coupled to an external wave guiding connection 36 by meansof which oscillatory energy can be led off for utilization.

The useful interaction between the electron flow and the 4traveling waveis secured along the constricted portion of the wave guiding passage 19.For providing cornponents having a velocity substantially equal to thatof electron flow, and considerably slower than the velocity oflightthere is formed along this portion an interdigital structure filter-typecircuit of the kind which is a principal feature of the invention. Thiscircuit comprises, as nger elements, a linear array or succession ofwire loops 31, having a hairpin or U-shaped coniiguration. The lineararray includes two sets or rows 32 and 33, each of the rows including aseries of uniformly spaced elements 31 extending inwardly from one oftwo opposite side wall surfaces of the wave guiding passage 19. The tworows are interleaved in an interdigital pattern so that alternateelements of the linear array extend from the same one of the twoopposite Side walls, while adjacent elements extend from opposite sidewalls. The lengths of the elements are adjusted to be more than half ofthe spacing between the opposite surfaces so that a region 34 ofoverlapping exists as shown by the transverse view of Fig. 2. Thevarious tube elements are advantageously arranged yso that the electronstream ows through this region of overlapping. In particular, it isadvantageous to employ a cylindrical electron stream of circular crosssection which is adjusted to be enclosed substantially completely withinthis region 34. Suitable stream collimating means, such as an externallypositioned solenoid 35 to furnish a longitudinal magnetic field, servesto keep the electron flow straight. Cylindrical electron streams ofcircular cross section are well adapted for Brillouin-type electron owwith its consequent economy and simplicity in magnetic focussing. Animportant advantage of this form of interdigital circuit -is this readyadaptability Ato Brillouintype ilow, of whose principles adescriptioncan be found in the copending application Serial No. 168,202,tiled June l5, 1950, by C. C.y Cutler.

Moreover, it is generally advantageous to provide a uniformtransition'between the loaded portion of the waveguiding passage A19along which extends the interdigital circuit and the tuiloaded portionthereof. To this end, the length of the iinger elements 31 decreasesgradually from a uniform maximum along the major portion of theconstricted passage to a minimum at the upstream end of the passage toeifect a gradual reduction in the loading. This region of taperedloading acts primarily as a transformer section rather than as a portionof the interaction circuit since generally therealong there will not becomponents having an axial velocity sufliciently slow for streaminteraction.

lt also will generally be advantageous to employ finger lengths for theinteraction circuit shorter than a quarter of the guide Wavelength ofthe desired oscillatory frequency because at frequencies at which thelingers are a quarter wavelength they form a series of resonators andthe propagating properties of the circuit are thereby adverselyaifected. However, it is possible to 'achieve satisfactory operation inthe frequency band at which the length of the iinger elements is'between successive vodd numbers of quarter guide wavelengths inthisband.

It is characteristic of this interdigital circuit that the normalelectric mode of waves propagating in the passage 19 will be distortedby the nger elements and axial electric field components will existbetween adjacent elements having a strong spatial harmonic componentwith a fundamental periodicity given by the spacing between successiveelements of one set. Electrons moving axially and close to the elementswill move in a substantially unidirectional iield provided they takeabout one half a period of the propagating wave to move from one elementto an adjacent one assuming that the guide wavelength is long comparedto the spacing between adjacent elements as is generally desirable. Thisis true whether electrons and the propagating waves are moving either inthe same or opposite directions.

For backward wave type oscillations, the electron stream is made tointeract with a particular backward or loppositely directed travelingwave. Such a wave of low ampltiude is set up initially by noisecomponents of the stream. For the amplication of this wave sutcient tosustain oscillations, the velocity of the electron stream is adjusted tobe substantially equal to the phase velocity of a forward travelingspatial harmonic of this particular backward traveling wave. It can beshown that for oscillations, a relationshipto be satisfied is that wherev is the average or D.C. velocity of the electron stream past theinteraction circuit, w is the radian frequency of the wave to beamplified, d is the spacing between adjacent finger elements, 0 is thephase shift between adjacent iinger elements, and n is an integer. Thevelocity v of the electron stream is primarily determined by theaccelerating voltage acting thereon, which is the potential differencebetween the cathode 13 and the structure 16. Lead-in conductors suppliedfrom a suitable voltage source provide the necessary voltages on theseelements. Additionally, both the spacing d and the phase shift 0 can beadjusted as desired by varying the geometry of the wave guiding passage,and the dimensions and positioning of the finger elements.

Alternatively, if one pair of adjacent finger elements is considered asa single iterative section, the relationship to be met is that where Lis the length of one iterative section (twice d) and qs is the phaseshift along each section of the wave to be amplified (twice 0).

It is also necessary in order to initiate and snstainoscillations thatthe beam current be sufficient to exceed a certain starting current. Tothis end, it is merely necessary to increase the beam current untiloscillations begin,

The intensity Vof'this beam current is determinedl primarily by thevoltages applied by lead-in conductors from a suitable Voltage source tothe cathode 13 and the beam forming and intensity control electrode 14.It is essential, of course, to feed back energy to sustain oscillations.This feedback is automatically supplied by the electron stream itselfwhich is traveling in a direction opposite'to the direction of the wavepropagation and interacting therewith. For a more detailed descriptionof the principles of backward wave oscillators, reference is Vmade to mycopending application, Serial No. 288,437, filed May 17, 1952.

It will be helpful at this point to describe briefly, by way of example,a method of construction found suitable for a hairpin circuit of thekind shown, designed for incorporation in a tube to have a centerfrequency of 50,000 megacycles (6 millimeters wavelength) for operationover a band 20,000 megacycles wide. The hairpins are made of gold-platedmolybdenum wire of 2 mils diameter. This wire is wound by means of aprecision helix-winding machine on a steel mandrel of lsubstantiallyrectangular cross section, approximately 8 mils by 80 mils, having itssharp edges beveled smooth. The pitch of the winding along the mandrelis adjusted to be 10 mils for providing a center to center spacing ofadjacent fingers of 5 mils, when two such windings are interleaved toform the interdigital circuits of Figs. 1 and 3. The mandrel, with thewire helix, is then inserted in a longitudinal groove in a copper piecewhich will form one of the two surfaces of the wave guiding passage fromwhich a row of elements will extend. The depth of the groove is adjustedso that the height of that portion of the mandrel extending above theface of the copper piece, and consequently the height of the helixportion extending above that face corresponds to the desired fingerlength. The wire is then brazed to the edges of the groove which aregold plated to facilitate this end. A taper may be ground at either orboth ends of the wound helix as desired in order to provide taperedsections at the ends of the interdigital circuit as shown in Figs. 1 and3. The steel mandrel is then dissolved chemically, leaving the helixportions projecting from the copper piece to form a-succession ofU-shaped or hairpin-shaped finger elements. This process is repeated toobtain a second copper piece from which projects a similar row ofhairpins. The two copper pieces are properly aligned so that the tworows of hairpins are interleaved and then brazed together with twocopper pieces which will form the two other sides of the wave guidingpassage.

Fig. 3 shows a modification of the tube shown in Fig. 1 moreparticularly designed for amplifier applications. It is convenient todesignate elements of this tube by reference numerals which exceed by100 corresponding elements of the tube shown in Fig. l. The principaldifference is the provision of a wave guiding path from the collector ordownstream end of the wave guiding passage 119 to an external couplingconnection 137, similar to the wave guiding path provided at theelectron source or upstream end of the passage 119 to the externalcoupling connection 136. This coupling connection 137v will furnish aninput wave which is to be amplified when the tube is operated as abackward wave amplifier, or alternatively there will be supplied theretoan output wave which has been amplified when the tube is operated as aconventional forward wave amplifier. Accordingly, the, main structure116 will include a section 140 at the collector end which defines a waveguiding passage 141 forming a continuation of the wave guiding passage119 to the tube envelope where a capacitive connection is made through'the envelope to the, external connection 137. As at the electron sourceend, the cross section of the wave guiding passage 119A graduallyincreases from the minimum portion along cent elements.

facilitating connection to an external circuit, a right angle bend 142Iis provided between wave guiding passages 119 and 141.

In this tube, it is also advantageous to decrease the loading graduallyat the collector end of the wave guid-v ing'p'ath 119 to make possible abroad band match. Accordingly, as at the electron source end, the lengthof the finger elements 131 is gradually decreased to effect a smoothtransition to a condition of minimum loading;

rIt is important when this tube is operated as an amplifier to insurethat the beam current does not exceed the value Which will initiatebackward type oscillations of the kind for which the tube of Fig. l isdesigned.

As has been indicated above, the tube shown in Fig. 3 can be adapted foroperation either as a backward wave amplifier or as a more conventionalforward wave arnplifier. For operation in the backward wave mode, thevelocity of the electron beam is adjusted to be equal to wad IL1r-0where wc is the radian mid-band frequency of the operating range, d isthe separation of adjacent elements, n is an integer, and 6 is the phaseshift between adja- Alternatively, for operation in a forward wave mode,the velocity of the electron stream is adjusted to be equal to wcdsummarizing, the present invention provides an improved form ofinterdigital filter-type wave circuit for use in various forms oftraveling wave tubes, particularly those intended for operation at veryshort wavelengths, where the tube dimensions are also necessarily short.It is to be understood that the specific embodiments described above areillustrative of the general principles of the invention. Various otherarrangements can be devised by one skilled in the art without departingfrom the spirit and scope of the invention.

What is claimed is:

l. In electronic apparatus, a hollow wave guiding member, a linear arraycomprising two sets of U-shaped wire loops, the closed ends of said wireloops of each set extending inwardly from and the open ends thereofbeing attachedV to a different one of two opposite surfaces of saidmember, the closed .ends of said two sets overlapping at the axis of thehollow wave guiding member and being interleaved in an interdigitalpattern, and means for forming and projecting an electron stream throughthe region of overlapping of adjacent wire loops.

2. In electronic apparatus, a hollow wave guiding member, coupling meansat one end of said member to be supplied with an input wave, couplingmeans at the other end of said member for removing an output wave, a

Vlinear array comprising two sets of spaced substantially U-shapedelements, the closed ends of the elements of each set extending inwardlyfrom and the open ends thereof being attached to a different one of twoopposite surfaces of said member, the closed ends of said two sets ofelements overlapping at the axis of said wave guiding member and beinginterleaved in an interdigital pattern, and means for forming andprojecting an electron stream through the region of overlapping ofadjacent elements.

3. In an oscillator, an elongated hollow wave guiding member, means forforming an electron stream for longitudinal traversal of said member, aplurality of substantially U-shaped wire elements spaced apart in alinear array in the direction of electron flow, the closed ends ofalternate elements extending from and the open ends thereof beingattached to one surface of said member, the closed ends of adjacentelements extending from and the open ends being attached to oppositesurfaces of said member in an nterdigital pattern with the closed endsoverlapping in the path of electron. flow,.means for terminating thedownstream end of said member -to be substantially reflectionless, and ahollow wave guiding member coupled to the upstream end of the elongatedhollow wave guiding .member for abstractingroscillatory energy.

4. In an oscillator, an elongated hollow wave guiding member having anopen end and an opposite end which is terminated to be substantiallyreilectionless, means for forming an electron stream for projectionthrough said member from said open end towards said opposite end, alineas array of U-Shaped wire loops spaced apart in the direction ofelectron liow, the closed ends of alternate wire loops of said arrayextending from and the open ends thereorC being attached to the sameside of said member, the closed ends of adjacent wire loops of Saidarray extending from and the open ends thereof being attached toopposite sides of said wave guide member in an interdigital pattern withthe closed ends of said wired loops overlapping inthe path of electronow; and means for connectingJr said elongated hollow wave guiding memberto a second hollow wave guiding member at the open end of the elongatedmember for abstracting oscillatory energy for utilization, saidlast-mentioned means including a transition region wherein the elongatedhollow member is gradually increased in its dimensions to merge into thesecond hollow member.

5. In an oscillator, an elongated hollow wave guiding member having oneend closed and the opposite end open, dielectric means coated withllossy material adjacent the closed end for providing a reflectionlesstermination, means for projecting an electron stream through the hollowof said member from the open end towards the closed end, a plurality ofsubstantially U-shaped wire elements spaced apart in -a linear array inthe direction of electron liow, the cloud ends of alternate elementsextending from and the open ends thereof being attached to the same sideof the member, the closed ends of adjacent elements extending from andthe open ends thereof being attached to opposite sides of the member inan interdigital pattern with the closed ends of said wire elementsoverlapping in the path of electron flow, and a second hollow waiveguiding member coupled through said open end to the elongated member`for abstracting oscillatory energy therefrom.

6. An oscillator according to claim in which the electron stream has avelocity which is equal to where w is the oscillatory radian frequencyof operation, d is the spacing between adjacent elements, n. is aninteger and 0 isthe radian phase shift between adjacent elements.

7. Electronic apparatus for amplifying electromagnetic waves having amidband radian frequency w comprising an elongated hollow wave guidingmember, means for forming and projecting an electron streamlongitudinally through the hollow interior of said member, a hollow waveguide to be supplied with an input wave to be amplified coupleddownstream along said member, -a hollow wave guide for abstracting theamplified wave coupled upstream along said member, and a linear array ofhairpin-shaped wire elements spaced along said member, the closed endsof alternate elements extending from and the open ends thereof beingattached to one side of said member, the closed ends of adjacentelements: extending from and the open ends thereof being attached toopposite sides of said member in an interdigital pattern with the closedends of said elements overlapping in the path of electron liow, thevelocity of the electron fiow being equal to WIL-0 where d is thespacing between adjacent elements, 'n is an integer, and@ is the radianphase shift betweenadj'a-f cent elements.

8. In a device which utilizes the interaction between an electron streamand a traveling electromagnetic wave for amplifying the wave, means for4projecting an elecof its length on the side of the stream opposite tothe first conductive surface, and a second plurality of -substanti'allyU-shaped wire loops extending from and attached to the second conductivesurface .in a manner to have their closed ends encircle the stream, theU-shaped ywire loops of said first .and second plurality 'beinginterleaved in the direction of the electron stream to forman'interdigital circuit.

V9. In a device which utilizes the interaction between an electronstream and a traveling electromagnetic Wave for amplifying the wave,vmeans lforming a hollow conductive wave guide, means for projecting anelectron stream along the axis of said Vwave guide, a rst plurality ofVsulxstantially U-shaped wire loops each having its open end vfixedy yto'one side ofthe interior surface of said conductive wave guide andextending into the hollow waveguide to have its closed end encircle theelectron stream, and `a second plurality of substantially U-shaped wireelements each having its open end fixed to the opposite side of theinterior surface of the conductive waveguide and extending into thehollow wave guide to have its closed end encircle the electron stream toform an interdigital circuit.

10. In a device which lutilizes the interaction between an electronstream and a traveling electromagnetic wave for amplifying the wave, atransmission line for propagating an electromagnetic wave in apredetermined direction including a first conductive surfaceextending'in a direction parallel to the direction of wave propagation,

a second conductive surface extending in a direction.

substantially parallel to the first and spaced apart from each other ina direction transverse to thedirection of wave propagation, and an arrayof U-shaped conductive finger elements along the interspace between saidfirst and second conductive surfaces, said array comprising a lirstsuccession .of elements, the open ends of which are attached to thefirst conductive `surface and from which the closed ends extend morethan half way but less `than the entire Way to the secondV surface and asecond succession of elements, the open ends of which are attached tothe second surface and from which the closed ends extend more than halfway but less than the entire way to the lirst surface, the closed endsof said elements of the first succession being interleaved with those'of the second in an interdigital pattern; and means for projecting .-astream of electrons along an extended path .in Vcoup-ling proximity witha wave propagating along said circuit; said device being characterizedin that the finger elements of the Vfirst and second successions areapertured and the aperture in each is aligned with the electron streamfor passagetherethrough of said stream.

References Cited in the file of this patent UNITED STATES PATENTS2,566,087 Lerbs Aug. 28, 1951 2,622,158 Luca Dec. 16, 1952 2,643,353Dewey June 23, 1953 2,683,256 Kumpfer July 6, 1954 2,702,370 Lerbs Feb.15, 1955 2,708,236 Pierce May l0, 1955 2,768,322v Fletcher Oct. 23,Y1956

